RPM moves up and down at full speed?

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I was wondering if someone could help me out. I have a 96 s-10 blazer when you get up to a constant HWY speed or use the cruise control the RPM move up and down about 5000. It is only noticeable when you get to higher speeds 45MPH and up. I had a similar problem with a Dodge Durango and that problem ended up being a throttle position sensor? Has anyone else had a similar issue which could help me out or advice?

Thanks!